Re: [PATCH RFC] Revert "KVM: VMX: Micro-optimize vmexit time when not exposing PMU"

["Xu, Like" <like.xu@intel.com>]

On 2020/6/19 17:40, Vitaly Kuznetsov wrote:
> Guest crashes are observed on a Cascade Lake system when 'perf top' is
> launched on the host, e.g.
Interesting, is it specific to Cascade Lake?

Would you mind sharing the output of
"cpuid -r -l 1 -1" and "cat /proc/cpuinfo| grep microcode | uniq" with us ?

Thanks,
Like Xu
>
>   BUG: unable to handle kernel paging request at fffffe0000073038
>   PGD 7ffa7067 P4D 7ffa7067 PUD 7ffa6067 PMD 7ffa5067 PTE ffffffffff120
>   Oops: 0000 [#1] SMP PTI
>   CPU: 1 PID: 1 Comm: systemd Not tainted 4.18.0+ #380
> ...
>   Call Trace:
>    serial8250_console_write+0xfe/0x1f0
>    call_console_drivers.constprop.0+0x9d/0x120
>    console_unlock+0x1ea/0x460
>
> Call traces are different but the crash is imminent. The problem was
> blindly bisected to the commit 041bc42ce2d0 ("KVM: VMX: Micro-optimize
> vmexit time when not exposing PMU"). It was also confirmed that the
> issue goes away if PMU is exposed to the guest.
>
> With some instrumentation of the guest we can see what is being switched
> (when we do atomic_switch_perf_msrs()):
>
>   vmx_vcpu_run: switching 2 msrs
>   vmx_vcpu_run: switching MSR38f guest: 70000000d host: 70000000f
>   vmx_vcpu_run: switching MSR3f1 guest: 0 host: 2
>
> The current guess is that PEBS (MSR_IA32_PEBS_ENABLE, 0x3f1) is to blame.
> Regardless of whether PMU is exposed to the guest or not, PEBS needs to
> be disabled upon switch.
>
> This reverts commit 041bc42ce2d0efac3b85bbb81dea8c74b81f4ef9.
>
> Reported-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> - Perf/KVM interractions are a mystery to me, thus RFC.
> ---
>   arch/x86/kvm/vmx/vmx.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 36c771728c8c..b1a23ad986ff 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6728,8 +6728,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>   
>   	pt_guest_enter(vmx);
>   
> -	if (vcpu_to_pmu(vcpu)->version)
> -		atomic_switch_perf_msrs(vmx);
> +	atomic_switch_perf_msrs(vmx);
>   	atomic_switch_umwait_control_msr(vmx);
>   
>   	if (enable_preemption_timer)